Images may be created by artists and technicians, using various techniques, from an actual physical scene, from an imaginary one that is originated within the mind of its creator, or from a scene generated by a computer. Still photographs of a real scene may be captured in a particular fashion, including filtering, to achieve the artist's expression and evoke some emotional response in the viewer. The creation of an image by hand may include very basic methods of manipulating perception, such as the creation of the illusion of a three dimensional perspective view on a flat sheet, which may be accomplished using a one-point perspective, or a two-point perspective, etc. The creation of an image by computer may be created by means such as the three dimensional rendering of mathematic scene description, e.g., a rendering of a fractal landscape or interactive virtual game world. The ability of an artist to use a computer to manipulate an image that has been captured digitally to manipulate the viewer's perception, may be just as powerful, even more powerful.
It is now known that a key basis for more complex manipulation of the way a viewer perceives an image is through the particular use of spatial frequencies therein. In physics and mathematics, a spatial frequency is the measure of how often a cyclic component is repeated per unit of distance. In wave mechanics, the spatial frequency (v) is related to the wavelength (λ) by the formula, (v)(λ)=1 (or, v=1/λ). In discussions of the human visual system, “Spatial frequency” refers to the number of pairs of bars imaged for a given distance on the retina, and may be expressed as the number of cycles per degree of visual angle. One-third of a millimeter is a convenient unit of retinal distance because this image size may subtend one degree of visual angle on the retina. The use of different spatial frequencies conveys different information to a viewer about various aspects of an image. High spatial frequencies represent abrupt spatial changes, such as edges formed within the image, corresponding to information regarding particular features and fine detail, whereas low spatial frequencies represent global information relating to overall shape, particularly its general orientation and proportions.
Visual information at different spatial frequencies or ranges of spatial frequencies may be utilized by an artist, and may be layered one upon the other, to convey different impressions and features, which is because different parts of the human eye are more perceptive to certain spatial frequencies. With an image being composed of layers of both high and low spatial frequency patterns, what is perceived by a viewer will depend upon how the viewer looks at the image. The anatomical basis for this phenomenon is the adaptation of a person's central vision to seeing the finer details described by high spatial frequencies, but not the broad, blurry patterns described by low spatial frequencies, whereas a person's peripheral vision is the opposite, and is adapted to better seeing the low spatial frequencies, which must be visualized outside of your central line of vision.
These anatomical characteristics of the human visual system explain the mystery behind the smile of Leonardo Da Vinci's subject in the painting, “Mona Lisa.” The portion of her smile that appears happy (corners of the lips drawn upwards) is found within low spatial frequencies, and may thus be properly seen through a person's peripheral vision—when not looking directly at her lips and mouth. When attempting to look directly at Mona Lisa's smile, portions of the smile blend into the background, diminishing the appearance of a smile, leaving the viewer to question whether or not she is truly smiling. The same phenomenon may be experienced with other well known works of art, such as those of the impressionist painters, where a close and direct examination of those works makes it more difficult to recognize detail because the prolific low spatial frequencies of the work are essentially being viewed using a high-frequency filter, which conversely improves when viewing the work off-center and from farther away using peripheral vision—a low-frequency filter.
Therefore, the methods of the present invention are directed to manipulating both the high and low spatial frequencies of an image, to achieve certain desired simplification to, and stylization of, that image.